The invention relates to an apparatus for pulling crystals with a rotating pulling shaft, preferably from the melt, especially a monocrystal pulling apparatus.
In the crystal growing field a great number of different processes are known, for example crystal growing from the gas phase, from the solution or from the melt. The various processes for crystal growing from the melt have attained a leading position over other growing methods on account of their greatly developed technology and productivity.
In crystal growing from the melt, a great variety of crucible processes are being used. For example, there is the so-called Kyropoulos process, which is characterized by the immersion of a chilled seed crystal in the melt. There is also the so-called Czochralski process in which a crystal is pulled from the melt. Also there is the Bridgman process, which is characterized by a vertical lowering of a crucible in the temperature gradient. Lastly, there is vertical zone melting without a crucible.
The subject matter of the invention is widely applicable in all crystal pulling apparatus with rotating pulling shaft, in which rotary vibrations can occur in the area of the thin-necked pulling shaft crystal, or in which there is danger that the crystal may break during the pulling process. These rotary vibrations occur on account of the elasticity of the so-called "thin neck."
In Leybold's publication No. 45-100.02, entitled "Crystal Growing," apparatus and processes for pulling crystals from the melt are described. This state of the art can serve as the starting point of the present invention.
FIG. 9 of Leybold's prospectus shows a production system with a rotating pulling shaft which permits a very stable crystal pulling process, especially at the higher rotatory speeds of the shaft.
The subject matter of the present invention has to do exclusively with the pulling shaft system.
In such systems the following disadvantages occur in practice:
Rotary vibrations occur between the pulling shaft and the growing crystal due to torsion in the elastic "thin neck," which is in the hot state. This is especially the case as the weight of the pulled crystal increases. There are resonance phenomena which intensify the undesirable rotary vibrations. These rotary vibrations are superimposed on the crystal rotation. The crystal speeds ahead and lags behind the rotating pulling shaft. This interferes with the perfect, stable pulling process. These harmful rotary vibrations can increase to the point of breaking the thin neck, and allowing the crystal to fall into the melt.
The danger of breaking off the crystal exists also when the rotation of the pulling shaft is abruptly stopped due to a power failure.